The present invention relates to a method of making formed bodies from iron alloys, in particular for making cages for radial roller bearings, axial roller bearings or linear roller bearings.
Steel cages for roller bearings are manufactured on an industrial scale in various fashions. Examples include drawing, punching or bending steel sheet or also steel pipe, as described e.g. in German publications DE-OS 21 47 169 or 21 47 171. For realizing cages to withstand particularly high loads, the strength and wear-resistance of cages can be further improved through additional heat treatment or thermochemical treatment, such as casehardening or nitriding, as disclosed e.g. in German publication DE-OS 25 56 745.
It is also known to manufacture cages for radial bearings from a strip material by punching slots into the strip for formation of pockets that receive rolling elements, subsequently bending the strip into a round configuration, and optionally fusing adjacent ends of the strip, as disclosed e.g. in French Pat. No. 1,256,601 or U.S. Pat. No. 2,288,564. This type of manufacturing process based on a bent strip is also applicable for axial bearings, as described e.g. in German Pat. No. DE 21 46 056 C3.
U.S. Pat. No. 3,173,192 describes a process for manufacturing cages for a cylindrical roller bearing by initially forming a strip of non-hardened low carbon steel into a desired profiled configuration and subsequently punching a plurality of spaced slots into the metal strip to form pockets for receiving rolling elements. Thereafter, the thus prepared metal strip is bent into a round configuration to form a cage which then is exposed to a heat treatment for purposes of obtaining improved mechanical properties with respect to hardness, strength and wear resistance. The sequence of this process is selected in this manner because it allows application of only low forces for realizing the shaping and punching steps in view of the untreated metal strip.
The application of heat treatment or thermochemical treatment, in particular in conjunction with delicate cages, i.e. cages of slight wall thickness and web strength, results in a distortion of the shape and size, rendering in extreme cases such formed bodies useless. The distortion of the workpieces are caused during hardening by superimposition of a deformation caused by internal stress during heat treatment and the volumetric change during transformation.